The present invention relates to the field of radiowave burst transmissions, and more particularly to a method of ramping transmission power in wireless communications devices using burst transmission.
Wireless communications devices using burst transmission, such as time-division multiplexed cellular telephones, transmit during prescribed time-slots. The transmissions in most systems must be within a relatively narrow frequency range, sometimes referred to as a frequency channel. In order to avoid interference, these devices are required to limit their transmission power in nearby frequency channels. Under steady state conditions, this may be achieved using known techniques. However, a non-trivial portion of the burst transmission may not be under steady state conditions. Indeed, it is common for battery powered burst transmitters to power their transmitters only during the appropriate time slots, and to power their transmitters during other time slots. Thus, in order to transmit during the transmit time-slot, the transmitter must be changed from a low (or off) power level to the desired transmit power level. Likewise, the transmitter must be returned to the low power level near the end of the transmit time slot.
In most applications, this changing of the transmit power level must be accomplished very rapidly due to the short duration of the time-slots. However, rapid changes in transmit power levels tend to generate undesirable transient harmonics which cause transmit power leakage into nearby frequency channels. In the terminology of ANSI-136, this means that quick transmit level changes leads to undesirably large transient adjacent channel power levels.
In the prior art, two main approaches have been taken to limit transient adjacent channel power in burst transmission wireless communications devices. Both methods are responsive to the design pressures present in battery powered wireless communications devices, including: 1) the power amplifiers used should operate at a point of high-efficiency; 2) the transmit modulator and other circuitry must consume as little power as possible; and 3) the complexity of all circuitry should be minimized to reduce cost. The first approach uses Finite Impulse Response (FIR) filters that are designed to reach peak output after a period of time that corresponds to the allowed ramping period for transmit power. At the start of each transmit burst, the FIR filters are reset and allowed to ramp-up naturally, typically resulting in a raised-root cosine response. It should be noted that the raised-root cosine impulse response may be specified by the relevant communication standard, such as ANSI-136, and other communications systems may use other low-pass impulse responses. The second approach relies on forced linear ramping using gain control of the power amplifier or the RF modulator. This linear ramping gain control requires additional complexity and may actually increase transient adjacent channel power due to signal discontinuities.
Thus, there remains a need for an improved transmit power ramping method for burst transmissions. Such a method should reduce adjacent channel power without requiring complex circuitry or high power drain.
The present invention applies an artificial ramping waveform profile to the power amplifier in order to reduce transients. An artificial ramping profile source is supplied with a plurality of pre-determined artificial ramping profiles. At the beginning of the ramping period, one of the artificial ramping profiles is selected and fed from the artificial ramping profile source to the power amplifier. The selection of the artificial ramping profile is based at least in part on the first message symbol of the message to be burst transmitted. Preferably, each different possible first message symbol has its own unique corresponding artificial ramping profile, and the corresponding waveform is used to artificially ramp the power amplifier. For instance, if the first message symbol is Type X, then corresponding artificial ramping profile X is selected; if the first message symbol is Type Y, then artificial ramping profile Y is selected, and so forth. At the end of the ramping period, the inputs to the power amplifier are switched to the traditional signal source, such as the FIR filters, etc., for receipt of the message symbols. The transmitted signal is based then on the response of the power amplifier to the artificial ramping profile and the message symbols.
In some embodiments, the selection of the artificial ramping profile is not based only on the first message symbol to be transmitted, but also on the next message symbol, or on the next several symbols.
The generation of transient adjacent channel power may be significantly reduced by artificially ramping the power amplifier rather than allowing for natural ramping of the power amplifier. By choosing the artificial ramping profile based on the first message symbol, the phase trajectory changes near the end of the ramping period may be significantly smoothed, thereby lessening the generation of undesired harmonic power levels. Further, this may be accomplished with reduced bias current in the power amplifier, thereby saving battery power, and without the additional circuit complexity associated with forced linear ramping.